FIG. 1 is a block diagram of a receiver in a general OFDM system, including an analog-to-digital converter (ADC) 110, a symbol start detector 120, a fast Fourier transform (FFT) window position controller 130, and a fast Fourier transformer (FFT) 140.
First, considering a symbol of an OFDM signal, when N subcarriers are used in an OFDM system, the symbol is comprised of N useful data samples as the output of an inverse fast Fourier transform (IFFT) for transmission, and a guard interval having G sample lengths to be inserted before a useful data section to prevent interference between symbols.
Here, the guard interval copies the end portion of the useful data section. A transmitter (not shown) adds G complex values to N complex values output by an inverse fast Fourier transformer (IFFT), and sequentially transmits a symbol comprised of a total of (G+N) samples.
Here, the guard interval is generally longer than a delay spread time of a channel. For example, a European digital TV broadcasting standard defines a guard interval having a length of ¼, ⅛, 1/16 or 1/32 (hereinafter, called a ¼ mode, a ⅛ mode, a 1/16 mode, and a 1/32 mode) of an actual symbol length. A transmitting side selects a length among these and uses the selected length. A receiver must perform accurate time synchronization to recover a received OFDM signal. The time synchronization is comprised of FFT window position recovery for parallel processing of an accurate signal, and sampling clock recovery for controlling a sampling clock of a received signal having a maximum signal-noise-ratio (SNR).                               s          j                =                                            ∑                              n                =                                  -                  G                                                            N                -                1                                      ⁢                          x                              j                ,                n                                              =                                                    ∑                                  n                  =                                      -                    G                                                                    -                  1                                            ⁢                                                ∑                                      k                    =                    0                                                        N                    -                    1                                                  ⁢                                                      X                                          j                      ,                      k                                                        ⁢                                      ⅇ                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                                                                        π                          ⁡                                                      (                                                          N                              +                              n                                                        )                                                                          /                        N                                                                                                                  +                                          ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                ∑                                      k                    =                    0                                                        N                    -                    1                                                  ⁢                                                      X                                          j                      ,                      k                                                        ⁢                                      ⅇ                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              kn                        /                        N                                                                                                                                                    (        1        )            
Equation 1 expresses a j-th symbol comprised of a useful interval and a guard interval which are output by an IFFT (not shown) of a transmitter. Here, j denotes a symbol number, k is a carrier index, N is the number of useful data samples, n indicates a sampling time, and X(•) and x(•) respectively denote an input complex value and an output complex value of the transmission IFFT. In the right side of Equation 1, the first term is a guard interval portion and the second term is a useful data portion.
As shown in FIG. 1, the ADC 110 samples a received OFDM signal. The symbol start detector 120 detects information on a start portion of a symbol using the type of guard interval and the sampled OFDM signal. The FFT window controller 130 designates an FFT window point in time to activate the useful data portion of the FFT 140 using length information on the guard interval and length information of the symbol start portion detected by the symbol start detector 120. However, a device for detecting information on the length of the guard interval has not been developed up to now, and thus correct operation of the symbol start detector 120 and the FFT window controller 130 cannot be ensured.